1. Field of Invention
The invention relates to digital-to-analog conversion, particularly to digital-to-analog conversion operations performed simultaneously with shared converter components.
2. Description of Prior Art
Digital-to-analog (D/A) conversion is the process of converting a digital number value to an analog signal value. D/A conversion is an important feature of many digital systems that control, communicate through, or otherwise interact with a non-digital environment.
Important characteristics of D/A converters include precision, number mapping, conversion range, and conversion speed. The precision of a D/A converter is often measured by the number of bits allowed for an input digital number. Four-bit converters are generally considered to be of low precision, eight-bit through twelve-bit converters are generally considered to be of moderate precision, and sixteen-bit or eighteen-bit converters are generally considered to be of high precision. Low precision D/A converters tend to have low implementation cost and can be very fast. High precision D/A converters tend to have high implementation cost and may be very slow.
The number mapping of D/A converters is usually a uniform mapping, with analog steps of equal size corresponding to each increment in digital number value. However, it is not necessarily the case that the mapping is uniform. Potential deviations include nonlinear output, non-monotonic output, scale error, and output offset.
With a uniform linear mapping, the precision and the conversion range determine the smallest output variation which can be produced by a D/A converter. Often, D/A converters are used to drive electrical, mechanical, or optical elements which require a large output range, such as radar emitters, acoustic speakers, and optical fiber lasers.
Finally, the conversion speed is the rate at which a single conversion of a digital number value to an analog signal value can be completed. The conversion speed limits the rate at which a D/A converter can be re-used for multiple conversions and may also limit the bandwidth of analog signals in the application for which the D/A converter is used.
There are several prior art techniques for D/A conversion. These typically fall into one of two classes of D/A converters, which are instantaneous converters and time-averaging converters. A discussion of the principal varieties of each class appears below, with material coming largely from the discussion in the second edition of “The Art of Electronics” by Paul Horowitz and Winfield Hill.
One simple form of instantaneous D/A conversion uses a tree of scaled resistors selectively tied to a summing junction. The summing junction is the input of an op-amp, with the op-amp output proportional to the sum of input currents at the summing junction. The current through each resistor is equal to voltage across the resistor multiplied the inverse of the resistance value. Each bit of a digital number to be converted controls the voltage applied to each resistor. The most significant bit is associated with the smallest resistor value, while the least significant bit is associated with the largest resistor value.
A major drawback to scaled resistor D/A conversion is that there must be a wide range of possible resistor values with tight tolerances on the variation allowed for large resistors. This is a particular drawback for high-precision D/A converters. A major advantage to scaled resistor D/A conversion is that the analog output is available quickly once the bits of the input number have been applied.
A second form of instantaneous D/A conversion uses a ladder of resistor values. An R-2R converter requires only two resistor values rather than a wide range of resistor values. The analog output is available immediately on application of the bits of the digital input number.
A third form of D/A conversion uses frequency-to-voltage (F/V) conversion. A F/V converter is most useful when the digital input comprises a train of digital pulses rather than, for instance, a binary twos-complement number representation. The digital pulses are converted directly to analog values by averaging them using a low-pass filter. The averaging requires some time, so the analog output of the D/A converter is not available immediately.
A fourth type of D/A converter uses pulse width modulation (PWM). In such a converter, the digital input is used to adjust the duty cycle of a pulse generator. For instance, a digital input number can be compared to an increasing count. As long as the count is less than the digital input number, a comparator output is in a high state. Once the count is greater than the digital input number, the comparator output falls to a low state. The counting process is repeated for each pulse cycle. To generate the analog output, the comparator output is averaged over one or more pulse cycles.
A fifth type of D/A converter uses an averaged rate multiplier circuit. A rate multiplier produces a sequence of digital pulses at a rate that is, on average, a multiple of a known base rate. The pulses are not necessarily periodic, which means that their rate must be averaged in order to produce the desired analog output. Typically, an averaged rate multiplier D/A converter relies on the load it is driving for the averaging.
A general drawback to all of the prior art D/A converters discussed above is that they accept one digital number value as input and produce one analog signal value as output. To perform more than one conversion, these D/A converters can be re-used in a serial manner, or else they can be replicated with the replicas used separately in parallel.
There are many applications in which it may be desired to perform large numbers of D/A conversions very quickly. An example is image display. Consider a display array comprising 640 by 480 display elements, each of which should have a brightness with 8-bit precision. Displaying one image on the display array requires 307200 D/A conversions. If the display array is used to show a video at 30 frames per second, the array requires 9216000 D/A conversions per second. Suitable D/A converters, if they were few in number, would have to be very fast, or, if they were slow, would have to be large in number.